This invention relates to optical switching, and in particular to an optical switch employing Micro-Electro Mechanical System technology (MEMS).
Networking technology has become pervasive in the last decade. It is now possible to transmit over six trillion bits per second over a single optical fiber. With these transmission rates, however, has come a bottle neck in the central office where switching is performed to transmit information arriving over one optical fiber onto another optical fiber and toward the desired location. One prior art solution is evidenced by networks in which two trillion bits per second of data arriving on an optical fiber are divided into one thousand parallel two gigabit per second data streams, or ten thousand parallel one hundred fifty-five million bit per second data streams. Each of these data streams is switched using conventional electronic switching technology and then forwarded on to the desired location.
Another well-known technique for accomplishing this switching involves optical cross-connect switches, also known as Optical-Electrical-Optical or OEO switches. In such switches information arrives in an optical form at the switch, it is then converted into electrical form, switched electronically, then converted back into optical form and transmitted on to the next switch. Such an approach has numerous advantages, in addition to using well-established electronic technology for performing the switching. Unfortunately, however, such switching is dependent upon the particular protocol within which the data is encoded, and is extremely expensive, complex and physically large in high data rate implementations, for example forty gigabits per second. Of course, significant electrical power is consumed, and the solution does not scale to changes in data rate, port density or protocol.
As a result of these disadvantages, a switching technology known as the photonic cross connect switch, also known as Optical-Optical-Optical or OOO switch has been developed in which all switching is performed optically, i.e. the data is never converted to electronic form. In the photonic switch, there is no need for conversion of the optical input signal into electrical form, or conversion of the switched electrical signal back into optical form. It offers faster operation, higher capacity and lower power consumption.
FIG. 1a illustrates a prior art MEMS photonic cross connect switch, referred to as PXC 2a, in which mirrors are used that scan in two axes. Multiple optical fiber inputs with an equal number of lenses produce an equal number of approximately collimated input optical beams. (The optical beams are said to be collimated when the diameter of the beams does not change appreciably as the beam propagates.) In the implementation depicted, the beams are produced by positioning the input fibers in a fiber block 10 with an associated lens array 11. Block 10 consists of a two-dimensional array of fibers with a polished end face appropriately aligned to the input lens array 12.
The two mirror arrays consist of an input array 12 and an output array 13. Each array includes movable mirrors which allow a beam from an input optical fiber to be reflected to a desired output fiber. Because each mirror is movable in two axes, the switching is achieved by directing the input beams into different output optical fibers. The mirror position, and therefore the switching operation can be controlled using several different techniques. In one implementation a voltage is used to generate an electrostatic force to rotate the switch mirrors. In another approach, a magnetic field is used to provide magnetic actuation, while torsion bars are used to provide a restoring force for the mirrors. In general, moveable mirror array 12 is populated with as many mirrors as there are fibers in the input/output fiber array. For clarity, however, only a few mirrors are shown in FIGS. 1a and 1b. For a more detailed description of the movement and control of the mirror arrays, see commonly assigned copending U.S. patent application Ser. No. 09/572,979, filed May 16, 2000, now U.S. Pat. No. 6,628,041 B2, entitled A Micro-Electro-Mechanical System (MEMS) Mirror Device Having Large Angle Out Of Plane Motion Using Shaped Combed Finger Actuators And Method For Fabricating The Same.
FIG. 1b illustrates another prior art MEMS PXC 2b, in which a mirror is used to fold the geometry and eliminate some of the components shown in FIG. 1a. In PXC 2b, the input beam passes through the lens array and is reflected by the movable mirror array 12 onto fixed mirror 16. Mirror 16 thus replaces the output moveable mirror array 13, the output lens array 14 and the output fiber array 15. Mirror 16 directs optical beams back to the moveable mirror 12, back through lens array 11, and back into the fiber array 10. Thus the fiber array now becomes both an input and an output array. The mirrored configuration shown in FIG. 1b has the advantage that, in principle, any fiber can be switched to any other fiber, so the fibers do not need to be divided into sets of input and output fibers.
One disadvantage of all of the systems described above is the inherent loss in optical signal power. This fact drives the need to amplify the optical signals. It is well known that in the course of passing through long optical fibers, optical components and switches, the signals are attenuated, and are attenuated differently depending on the nature of the fiber, components and switches, and the actual path that the signals take in the network. One solution to this problem has been to provide amplification of the signals at intermittent locations along the fiber.
FIG. 2 is a schematic illustration of such a typical prior art system employing two wavelength division multiplexing (WDM) optical switches, together with amplification along the fiber connecting the two switches. As shown in FIG. 2, the optical fibers terminate at demultiplexer 22 to which they are optically coupled. Demultiplexed optical signs are input to PXC 32, which may be of the type shown in FIG. 1a or 1b, or some other arrangement. The role of demultiplexer 22 is to take multiplexed signal carried by fiber 20 and break it apart into its constituent signals, for example individual data-bearing wavelengths. In so doing, PXC 32 may operate on each constituent signal independently, for example, passing certain signals on to DWDM transmission system 28, while passing (or receiving) certain other signals to (or from) another network component, for example, a digital cross connect 30 having an optical-to-electrical converter 31 at the interface with PXC 32. Importantly, PXC 32 operates on these signals in the optical domain (without first converting the signals to the electrical domain). Thus, PXC is a type of OOO switch. Digital cross connect 30 requires conversion of the signals from the optical domain to the electrical domain, and thus is a type of OEO switch. In any event, the function of adding or removing constituent signals performed by the PXC 32 as illustrated and discussed above is generally referred to as “grooming.” After grooming, those constituent signals which are to be passed to the DWDM transmission system are recombined into a composite or multiplexed signal by multiplexer 33 before being provided to optical fiber 23.
After an appropriate distance 34 (typically measured in miles), the signal(s) on the optical fiber 23 are supplied to a booster amplifier 25, a line amplifier 26, and a pre-amplifier 27 by which the signal(s) are amplified. Eventually the multiplexed signal(s) reach demultiplexer 36. The signal is then demultiplexed, and as previously described, grooming may take place via PXC 37, the constituent signals at this point are passed to yet another multiplexer 43 of DWDM transmission system 29, and so forth through the network. Although the prior art system shown in FIG. 2 does provide amplification, and enables the transmission system to maintain control over the amplifier operation, it may or may not account for the loss in the PXCs.
Regardless of the form of the switch—OOO or OEO—long haul optical transmission systems typically use wavelength division multiplexing (WDM) to carry many optical signals on a single fiber. One problem, however, with WDM systems is maintaining a uniform amplitude among the various signals muxed together into the composite WDM signal, that is assuring that all signals arriving at a particular location are equally powerful, or controllably intentionally of different power levels, depending upon the desired use. This makes it advantageous to controllably balance loss with gain in different optical network elements at the constituent signal level. If this is done by electronic amplification, more complexity and balancing is required to compensate for the electronics. It would therefore be desirable to provide a system in which electrical regeneration is avoided and controllable optical amplification is provided at the constituent signal level.
Another advantage of the invention is that the gain control may be done within a node and across multiple nodes via fiber optical links using control plane software. Typical control plane software can be generalized multiprotocol label switching (GMPLS). Using this kind of control, the amplifier gain can be used to equalize losses across an arbitrary number of switches and network elements based on power sensed at any point within the network